Patent literature (PTL) 1, for example, discloses an optical signal transmission system. The optical signal transmission system disclosed in PTL 1 transmits signal light emitted from a laser diode to an optical receptacle through a transmitting fiber, an optical connector, and a receiving fiber. PTL 2, for example, discloses an optical connector used in such an optical signal transmission system. PTL 3, for example, discloses an optical receptacle.
The optical connector disclosed in PTL 2 is a spatial coupling type of connector that couples the transmitting fiber and the receiving fiber via a lens. The lens is configured to form an image by reducing the exit face of the transmitting fiber in size to the entrance face of the receiving fiber. As a result, this configuration maintains an optical line even if the spot position of the signal light incident on the receiving fiber deviates slightly from the core center of the receiving fiber. Therefore, the numerical aperture (NA) of the receiving fiber needs to be larger than the NA of the transmitting optical fiber.
The optical receptacle disclosed in PTL 3 includes a lens and a light receiving element, focuses signal light, which is diffusely emitted from the exit face of the receiving fiber into space, onto the light receiving element with the lens, and subjects the signal light to photoelectric conversion. Here, a light receiving element with a small-diameter receiving surface is used. In particular, optical signal transmission systems have been expected in recent years to support transmission of high-definition video signals, for example, and are thus required to achieve high-speed communication at 10 Gbps to 40 Gbps. Therefore, the diameter of the receiving surface (receiving diameter) has tended to reduce in size, since a smaller size achieves a good frequency response speed, with a receiving diameter of approximately 50 μm being frequently used. Accordingly, the lens is configured to form an image by reducing the exit face of the transmitting fiber in size on the light receiving element. As a result, the NA of the light beam incident on the light receiving element is greater than the NA of the receiving fiber.